Winterizing question

I'm installing a sprinkler system and have a question. I'm running my main trunk to the valve boxes from a pitless adapter from my well casing. From the pitless, I go to a shut off valve > sediment filter > then up through the frost line and into the trunk which is 18" below ground (above our freeze line). I was hoping to put a valve box above the shut off valve and sediment filter and let it hollow so I can access it when winterizing. However, even though it's below freeze line, since the pipes are exposed (some partially and some fully), wouldn't that create a freezing issue, even though they're below the freeze line?

Thanks for the info. I'm also little worried about my tee off of the casing. One side has a shut off which everything after that will be blown out. On the other side, it feeds the house. It will remain pressurized. All of this is below freeze line. I plan to cover this with a valve box and put insulation in the hole. However, the hole will be open for accessibility.